The present invention relates to a magnetic fluid bearing apparatus and a motor having a magnetic fluid bearing apparatus and, more particularly, to a magnetic fluid bearing apparatus employing a lubricating fluid such as a magnetic fluid and a lubricating oil and a motor having a magnetic fluid bearing apparatus in which no oil leakage is permitted to be discharged from a portion of the magnetic fluid bearing apparatus.
The magnetic fluid bearing apparatus as applied to the present invention comprises a magnetic fluid sealing member which is formed in a housing, with the magnetic fluid sealing member including a rotating shaft extending into the housing, a permanent magnet mounted on the housing, and a magnetic fluid arranged in a clearance between the rotating shaft and the permanent magnet and provided in the housing. A magnetic field circuit is constituted by the magnetic fluid sealing member comprising the rotating shaft, the magnetic fluid, and the permanent magnet.
The magnetic fluid sealing member having the magnetic fluid is combined with a sliding bearing mounted o the housing and surrounded at an outer peripheral portion of the rotating shaft.
The present invention relates to an electric motor having a magnetic fluid bearing apparatus, with such a motor being suitable for a motor for driving a polygon mirror for a laser beam printer (LBP), for driving a magnetic disc in a magnetic disc apparatus, driving a photo disc in a photo disc apparatus, and a video tape recorder (VTR) etc.
Recently, for this type of motor having a magnetic fluid bearing apparatus, it is required that the motor have a high accurate rotation with a low shaft deflection and a low rotation number fluctuation, and with a high speed rotation for attaining a highly accurate picture quality and a high density for a recording.
Further, for this type of magnetic fluid bearing apparatus use in the motor, it is desirable to maintain a clean magnetic fluid bearing apparatus in which no contamination by an oil vapor discharged from the lubricating fluid etc. occur.
The problems relating to rotation accuracy and contamination of this type of motor are caused mainly by the bearing apparatus structure. Conventional ball bearing apparatus structure have limitations for attaining the high speed rotation and the high rotation accuracy in the motor due to processing accuracy in manufacturing the ball bearing apparatus structure components.
For these reasons as a bearing apparatus structure in the motor for obtaining a high speed rotation and a high accuracy rotation, a sliding bearing apparatus structure using a lubricating fluid is employed, and there are many improvements for use in the motor having the above stated sliding baring apparatus structure.
In particular, as the countermeasures for the oil leakage from the sliding bearing apparatus, a magnetic fluid sealing structure is employed in which a sealing structure for the oil can expect to carry out a complete sealing
A magnetic fluid sealing structure of the aforementioned type in a motor having a magnetic fluid bearing apparatus comprises non-magnetic housing, a permanent magnet or a permanent magnet having a magnetic pole piece at an end face thereof is inserted into an opening end side portion of the housing, with the permanent magnet or the permanent magnet having the magnetic pole piece being arranged along an axial line of the housing, and with a permeable rotating shaft extending through the housing and arranged along the axial line of the housing. A lubricating fluid, such as a magnetic fluid, is arranged in a clearance formed between the rotating shaft and the permanent magnet or the permanent magnet having the magnetic pole piece.
The enclosed magnetic fluid seals the clearance between an outer peripheral portion of the rotating shaft and an inner peripheral portion of the permanent magnet or the permanent magnet having the magnetic pole piece. In accordance with the above stated magnetic fluid sealing structure comprising the rotating shaft, the permanent magnet or the permanent magnet having the magnet pole piece and the magnetic fluid, a magnetic field circuit is constituted in the housing of the motor.
Further, air in the housing is cubically expanded due to a heat generation in the bearing surrounding portion or due to a heat generation caused by the drive of the motor etc., thereby increasing an inner pressure in the housing.
With an increase of the inner pressure in the housing, a fluid film of the magnetic fluid, arranged between the outer peripheral portion of the rotating shaft and the inner peripheral portion of the permanent magnet, or the magnetic fluid sealing structure is destructed, the magnetic fluid scatters in the housing such as the bearing surrounding portion etc. resulting in a contamination of the magnetic fluid bearing apparatus by the scattered magnetic fluid.
For solving the above stated problems, in, for example, Japanese Patent Laid-Open No. 210560/1986, a motor is provided having an intake or suction opening provided on a housing so as to prevent the increase in inner pressure in the housing. The magnetic fluid sealing structure for the motor is combined with a ball bearing apparatus structure and, therefore, it is impossible to obtain the motor having the high speed rotation and the high rotational accuracy due to the ball bearing apparatus structure.
In the above stated prior art, the intake or suction opening prevents the increase in inner pressure in the housing, however, since the oil vapor from the lubricating oil, enclosed in the ball bearing portion, escapes toward an outside potion through the opening of the housing an original function of the magnetic fluid sealing structure is adversely affected.
U.S. Pat. No. 4,526,484 also proposes a magnetic fluid bearing apparatus in which the magnetic fluid sealing structure and a sliding bearing apparatus structure employing a magnetic fluid as a lubricating fluid are combined.
In the aforementioned United States patent, the magnetic fluid (ferro-lubricant) extends throughout as a reservoir on either side of generally radial or cylindrical thrust bearing surface and also an annular ring about an annular permanent magnet as a thin liquid film along the radial bearing surface. Therefore, the magnetic fluid bearing apparatus has no space in a housing between an outer peripheral portion of a rotating shaft and an inner peripheral portion of the annular permanent magnet.
In this magnetic fluid bearing apparatus, a high accuracy speed rotation can obtain by the sliding bearing apparatus structure according to the fluid lubrication and the leakage of the lubricating fluid can be prevented according to the magnetic fluid sealing structure.
In the above stated magnetic fluid bearing apparatus having the sliding bearing apparatus structure, the lubricating fluid itself is cubically expanded in the housing by the increase in temperature and there is the possibility that the magnetic fluid overflows from the magnetic fluid sealing structure and flows toward an outside of the housing.
It is undesirable for magnetic fluid to overflow from the magnetic fluid sealing portion due to the cubic expansion in the lubricating fluid, since at the high speed rotation, the magnetic fluid may scatter from the magnetic fluid sealing structure due to an increase in the centrifugal force acting on the magnetic fluid at the magnetic fluid sealing structure. However, no consideration has been given to this particular pattern.